A grey wedge appears on the screen
You've set a View Clipping Border

View Clipping Border (⎇ Alt +B)
Allows you to define a clip/ping border to limit the 3D View display to a portion of 3D space. It can assist in the process of working with complex models and scenes.
Once activated with Alt-B, you have to draw a rectangle with the mouse, in the wanted 3D View. It becomes a clipping volume of four planes:
A right-angled parallelepiped (of infinite length) if your view is orthographic.
A rectangular-based pyramid (of infinite height) if your view is in perspective.
Once clipping is used, you will only see what’s inside the volume you have defined. Tools such as paint, sculpt, selection, transform snapping, etc. will also ignore geometry outside the clipping bounds.
To delete this clipping, press ⎇ Alt+B again.
The dark gray area is the clipping volume itself. Once clipping is deactivated with another Alt-B, all of 3D space will become visible again.

From the manual, that most ignore, but contains useful information for those that still remember how to read.
https://docs.blender.org/manual/en/2.79/editors/3dview/navigate/clip.html